The primary goal of this research proposal is to understand the impact of chemotherapy on muscle strength, and to test a novel therapeutic approach to improve muscle function in combination with chemotherapy. Peripheral muscle weakness is one of the most common side effects of cancer treatment and represents a prominent clinical problem. Functional deficits due to muscle weakness (i.e. impaired ability to generate force) can persist from months to years following remission and there is no effective therapy. One underlying mechanism of muscle weakness in cancer patients is an increase in oxidative stress due to certain chemotherapeutic agents. Our preliminary data shows a completely novel mechanism of skeletal muscle weakness in cancer via increased oxidative stress leading to intracellular calcium (Ca2+) leak in muscle that reduces muscle contractile force. The ryanodine receptor/Ca2+ release channel (RyR1) on the sarcoplasmic reticulum (SR), is a key protein involved in skeletal muscle excitation-contraction (E-C) coupling. Oxidation of RyR1 leads to Ca2+ leak and reduced tetanic Ca2+, which directly determines the force of muscle contraction. We have found that in breast cancer with bone metastases that increased NADPH oxidase 4 (Nox4) expression in muscle, a constitutively active oxidase that generates reactive oxygen species (ROS), leads to RyR1 oxidation and Ca2+ leak. Stabilization of RyR1 using the small molecule Rycal S107 (currently in phase II clinical trial for heart failure, reduces Ca2+ leak and restores skeletal muscle force in our model of cancer- associated weakness (without affecting tumor burden). Thus, blocking RyR1 Ca2+ leak in the presence of chemotherapy-induced oxidative stress may be an effective strategy to improve muscle function and patient outcomes and represents a completely novel approach in cancer patients. Our central hypothesis is that chemotherapy-induced oxidative stress targets RyR1 to promote sarcoplasmic reticulum Ca2+ leak causing and that blocking this Ca2+ leak using Rycal S107 in combination with chemotherapeutics will improve muscle strength while reducing tumor burden. Given the lack of effective treatment options and the lack of mechanistic understanding of chemotherapy-induced muscle weakness, this proposal aims to build on our data regarding oxidative stress-induced muscle weakness in cancer and address RyR1 Ca2+ leak as a novel therapeutic approach to improve muscle strength and reduce mortality in cancer. We will test our hypothesis using the following specific aims. Aim 1 will determine the impact of chemotherapy on skeletal muscle Ca2+ handling and function. We will measure forelimb grip strength and ex vivo skeletal muscle contractility in mice treated with chemotherapy and measure oxidative stress and muscle Ca2+ handling. Aim 2 will determine whether preventing RyR1 Ca2+ leak (using Rycal S107) in combination with chemotherapy reduces Ca2+ leak and improves muscle function.